1. Field of the Invention
The present invention relates to a fuel pressure controller of a direct injection internal combustion engine for controlling a discharge amount of a high-pressure pump, which supplies high-pressure fuel to an injector, through feedforward control and feedback control.
2. Description of Related Art
A period from injection to combustion of fuel is shorter in a direct injection engine that injects fuel directly into a cylinder than in an intake port injection engine that injects the fuel into an intake port. The direct injection engine cannot have a sufficient time for atomizing the injected fuel. Accordingly, the direct injection engine has to atomize the injected fuel by increasing injection pressure to high pressure. The direct injection engine pressurizes the fuel, which is drawn from a fuel tank with a low-pressure fuel pump, to high pressure and pressure-feeds the high-pressure fuel to an injector with a high-pressure pump driven by a camshaft of the engine. The direct injection engine senses pressure of the fuel (fuel pressure) supplied to the injector with a fuel pressure sensor and controls a discharge amount of the high-pressure pump (valve closing time of fuel pressure control valve) to conform the sensed fuel pressure to target fuel pressure.
A recent direct injection engine sets the target fuel pressure for each operation area and controls the fuel pressure in a wide range as shown in FIG. 4. In FIG. 4, Tr represents required torque and Ne is engine rotation speed. Thus, the fuel pressure is maintained at high pressure about 8 MPa even during an idling immediately before the engine is stopped. Therefore, the fuel leaking from the injector while the engine is not operating increasers. The leak fuel stays in the cylinder and is discharged at next engine start without being combusted. As a result, exhaust emission as of the start is deteriorated. As shown in FIG. 6, the leak fuel L increases as the fuel pressure P increases. Therefore, the leak fuel can be effectively reduced by decreasing the fuel pressure when the engine is not operating.
Generally, in the direct injection engine, the injector performs the injection two or three times for each fuel discharge from the high-pressure pump. There is a possibility that following performance of actual fuel pressure to follow a change in target fuel pressure cannot be ensured during a transitional period if the fuel pressure control (discharge amount control of high-pressure pump) is performed only through feedback control. Therefore, as described in Japanese Patent No. 3633388, feedforward control estimating and setting a control amount in accordance with a required fuel injection amount is used in addition to the feedback control setting the control amount in accordance with a deviation between the target fuel pressure and the actual fuel pressure. Thus, the following performance of the actual fuel pressure to follow the change of the target fuel pressure during the transitional period can be improved.
However, in this scheme, if the engine is stopped without performing the injection immediately after the high-pressure pump discharges the fuel corresponding to the two or three injections due to the feedforward control immediately before the engine stops, the engine is stopped in a state in which the fuel pressure P is increased largely by the discharge from the high-pressure pump immediately before the stopping of the engine as shown by a chained line in FIG. 7. In FIG. 7, ts represents the time when the engine stops. Thus, the leak fuel increases further.
As a countermeasure, JP-A-2005-133649 describes that a return pipe is connected to a delivery pipe, which distributes the high-pressure fuel to the injectors, through an electromagnetic relief valve. When an engine stop signal is detected, the electromagnetic relief valve is opened to return the fuel from the delivery pipe to the fuel tank through the return pipe, decreasing the fuel pressure.
However, since this structure requires the electromagnetic relief valve, a drive circuit of the electromagnetic relief valve and the return pipe, increase in a cost is inevitable. The high-pressure fuel in the delivery pipe is rapidly depressurized to proximity of an atmospheric pressure (pressure in fuel tank) and is returned to the fuel tank as the electromagnetic relief valve opens. Accordingly, vapor (air bubble) is easily generated in the fuel returned to the fuel tank. There is a possibility that the vapor is suctioned by the fuel pump at the next start.
JP-A-2004-293354 describes that the fuel injection is continued even after the engine stop condition is established. Then, the fuel injection is stopped to stop the engine when the actual fuel pressure decreases to the target fuel pressure. However, in this scheme, a delay is caused between the time when the engine stop condition is established and the time when the engine actually stops. Accordingly, there is a possibility that an operator feels discomfort.
As described above, in conventional technologies aiming to solve the problem of the oil-tightness in the system controlling the discharge amount of the high-pressure pump through F/F-F/B combination control using the feedforward control and the feedback control in combination, problems of the cost increase, the vapor generation and the delay in the engine stop timing can be caused.